The invention relates to a method of producing a gold-containing pigment from a gold compound that can be vaporized.
Gold-containing pigments have been used for a long time in the manufacture of red glazes, so-called ruby glazes. To this end, gold colloids (xe2x80x9ccassius gold purplexe2x80x9d), which were manufactured in an aqueous solution and stabilized with tin acid were melted into alkaline silicate glasses wherein, after tempering, the desired red color is formed (see Gmelin, manual of the inorganic chemistry, vol. 62 [Au], p. 399).
Ruby glazes are described in detail by W. Vogel, Glass-chemistry, Vol. 3, chapter 9.5, Springer Verlag 1992. Subsequently, the color formation process by tempering of ruby glazes is initiated wherein gold colloids precipitate and agglomerate. Also, the effects of protective colloid additives is discussed in this publication. These protective colloid additives have the effect that the tempering does not result in a complete deflocculation of the colloid particles formed. The protective colloid additives envelope the gold colloids if the number of the protective colloid particles is greater and their dimensions are smaller than those of the gold particles (envelope-protection). Tin acid sols in aqueous solutions have been found to be advantageous protective colloids for gold sols. In the red gold ruby glass, the gold colloid particles have a size in the range of 50 to 60nm.
DE 94 03 581 U discloses a coated nano-powder and an apparatus for the manufacture thereof. The coated nano-powder is manufactured in such a way that the core is formed in a first microwave plasma and the coating is formed in a second microwave plasma. The core may consist of a metal nitride or a metal oxide, whereas the coating may consist of an oxide, a nitride or a carbide of another metal.
DE 196 38 601 Cl discloses a method for the manufacture of particles with a core and an envelope. In this method, a vaporizable metal compound is introduced, together with a reaction gas, into a microwave plasma wherein the cores of the particles are formed. For the manufacture of the envelope of the particles, a second microwave plasma is not needed; rather, for providing the envelope, an organic compound, which can be polymerized, is polymerized on the core by the UV light of the microwave plasma. In this method, no material can be used for the envelope, which cannot be polymerized by exposure to light.
It is the object of the present invention to provide a method for the manufacture of gold-containing pigments without the use of aqueous chemical processes.
In a method of manufacturing a gold containing pigment, a vaporizable gold compound, a vaporizable compound of another metal which includes an oxide or a nitride with a melting temperature above the melting temperature of gold and a reaction gas consisting of a mixture of argon and oxygen are introduced into a first microwave plasma, whereby cores covered with gold clusters are formed from the oxide or nitride, and the gold cluster covered cores are subjected to a tempering process at 150xc2x0 C. to 300xc2x0 C. for causing coloration of the gold cluster covered oxide or nitride cores.
In accordance with the invention, gold-containing pigments can be synthesized in a microwave plasma apparatus as it is disclosed in the above-mentioned DE 94 03 581 U or by D. Vollath and K. E. Sickafus in xe2x80x9cNanostructured materialsxe2x80x9d, vol. 1, pp. 427-437, 1992. In such an apparatus, several microwave plasmas, for example, three plasmas, one disposed after another can be generated such that the reaction gas and the initial products and, respectively, intermediate products flow serially through the plasmas.
A vaporizable gold compound, a vaporizable compound of another metal and an oxygen and/or nitrogen-containing reaction gas are conducted into a first microwave plasma. As other metals especially such metals which form oxides or nitrides with a melting point above the melting point of gold (1064xc2x0 C.) are suitable such as zirconium, titanium, iron, chromium or manganese. As vaporizable metal compounds preferably the halogenides are employed. However, carbonyls or metal-organic compounds such as alkoholates also of gold and the other metal may be used. As reaction gas, a mixture of argon and oxygen is suitable for the manufacture of oxide cores. For the manufacture nitride cores hydrogen either together with nitrogen or ammonia, for example at a volume ratio of 4:1 or dry air resulting in oxide cores may be used. The gold compound and the compound of the other metal are introduced at a mole ratio of 1:1 to 1:1000. The reaction gas is used in a stoichiometric excess relative to the oxygen and/or hydrogen content.
As has been determined by an electron microscope examination, in this process step cores of the oxide or nitride of the other metal covered with gold clusters are formed without the need for a second microwave plasma. The cores are spherical and have a diameter of 5 to 10 nm. The gold clusters have a diameter of 1 to 1.5 nm.
The particles obtained after this step are light in color. They are for example white with a slight yellow or purple hue. They can be subjected subsequently to tempering at temperatures of between 150xc2x0 C. and 300xc2x0 C., preferably between 150xc2x0 C. and 200xc2x0 C., whereby the desired color is obtained.
Before the tempering step, the gold particles are stabilized preferably by tin oxide. This can be achieved for oxide cores by reacting the particles together with a vaporizable tin compound such as tin chloride in a subsequent second microwave plasma with the reaction gas which, in this case, contains oxygen. With nitride cores however, the stabilization must be obtained by an aqueous chemical process.
Before the pigment is melted into a glazing, the particles should be enveloped by a protective matrix. With oxide cores, this can be achieved in a subsequent third microwave plasma. In this case, oxygen-containing reaction gas is used, which does not include any other reactive components. As material for the protective matrix zirconium, ZrSiO4 is particularly suitable. The zirconium layer can be obtained in the microwave plasma with equi-molar amounts of zirconium chloride and silicon chloride. The production of the layer in the microwave plasma provides for relatively thin layers. If thicker layers are required, the thin zirconium layer produced with the plasma procedure can be reinforced by one of the aqueous chemical processes. The nitride cores must be coated also by an aqueous chemical procedure.
Below, the invention will be described in greater detail on the basis of examples.